The US presidential debate, a combat of titans pitting the Democrat Hillary Clinton against the Republican Donald Trump, is sure to be a major moment in the campaign, with just six weeks left before the Nov 8 election.

This first of three debates will give the candidates a chance to introduce themselves - or re-introduce themselves - to sceptical American voters who will be watching closely for the slightest misstep, awkward gesture or fatal altercation.

The debate could break all records for a US political audience, given the sharply contrasting personalities - and positions - of Trump and Clinton.

1. WHEN AND WHERE WILL IT TAKE PLACE?

The outside of Hofstra University's David & Mack Sport and Exhibition Complex, the location of the first presidential debate in Hempsted, New York.

Tuesday, 9am (Sept 27, Singapore time), at Hofstra University in the city of Hempstead on Long Island, an hour's drive from New York. Hofstra is no newcomer to presidential debates, having hosted them in 2008 and 2012.

2. WHAT IS THE FORMAT OF THE DEBATE?

90 minutes, in six 15-minute segments with no commercial interruptions.

Questions will focus on three broad themes: "America's Direction," "Achieving Prosperity" and "Securing America" . Three more questions related to news events that took place this week will also be put to the candidates.

The moderator will open each segment with a question. Each candidate will have two minutes to respond; each will then be allowed to reply to the other's response. The moderator will use the remaining time for follow-up questions.

Each broad theme will be discussed for 30 minutes.

3. WHO WILL BE THE MODERATOR?

Lester Holt is set to moderate the upcoming presidential debate between Hillary Clinton and Donald Trump.

Lester Holt, 57, the respected anchor of NBC's evening news, the country's most widely watched news programme. He moderated one of the Democrats' primary debates in January.

Holt has a tough task ahead, having come under scrutiny ahead of the debate, with the Clinton campaign and her Democratic supporters urging him to correct Trump if he makes false claims. There are also worries from her camp that he will toss simpler “softball” questions in Trump’s direction while pressing Clinton with a much more challenging interrogation.

Trump, meanwhile, has already stated that he does not believe Holt’s purpose as moderator is to police each candidate.

4. WHAT ARE THE STAKES FOR CLINTON AND TRUMP?

Large images of Democratic nominee Hillary Clinton and Republican nominee Donald Trump are seen on a CNN vehicle at Hofstra University.

Both Clinton and Trump enter the debate as the two most deeply unpopular presidential candidates in modern history. But the bar is higher for the Democratic candidate, given her experience and detailed knowledge of the issues. Both hope to discredit the other, and both hope to emerge from the debate having burnished the public’s view that they are better qualified to be commander-in-chief.

Clinton

Clinton will have to show that she is presidential but also honest (55 per cent of Americans do not think so), while proving that she has fully recovered from her recent bout of pneumonia. Clinton is not particularly well-liked, and anything she can do to create an emotional bond with voters would boost her cause.

David Kochel, a Republican strategist and former top adviser to Jeb Bush’s presidential campaign, has this advice for Clinton - be natural. “Hillary’s main goal is to be natural, authentic, and show humour without a script – be ‘likeable enough’. She’s a better debater than she gets credit for, but this debate will be far more about style than substance, because of Trump’s outsized presence on the stage," he said. “I would also have her prepped to needle at Trump’s wildly overstated wealth, as that seems to be the one thing that most easily flusters him."

Trump

Trump needs to convince voters that he has what it takes to be president, that he has at least an adequate familiarity with the issues and can make it through a high-pressure debate against a single opponent without losing his self-control.

"The key question is whether Clinton’s needling will get to him and whether he’ll revert back to early primary Trump and start up again with the nicknames and insults,” said Aaron Kall, director of debate at the University of Michigan and co-author of 'Debating The Donald'. "There are major concerns about his temperament to be president and that’s not the image he wants to project on stage while in front of undecided and sceptical voters who will soon be making their final decisions.”

Analysts say Trump will also have to reassure Republicans that although he is in many ways an outsider, he would serve as a Republican president.

5. HOW HAVE CLINTON AND TRUMP PREPARED?

Clinton

One of the biggest unknowns remains which Trump will show up. While Clinton has a lengthy record of meticulous preparation and formidable performances, Trump has been more unpredictable. Sometimes, he is the freewheeling showman prone to controversial utterances; other times, with help from his campaign team’s repackaging, he is a more sober and scripted candidate. To prepare for the debate, Clinton has been holding mock debate sessions where longtime aide Philippe Reines plays the role of Trump.

Trump

Trump aides said their candidate, who like Clinton participated in numerous TV debates during their respective parties’ nominating races, was preparing for the event but not doing mock debates where someone plays the role of Clinton. His chief handlers have mainly been focused for weeks on training him to control his penchant for drifting off message by booking events where he is trained to stick to the teleprompter.

Bloomberg reported that Trump also put in some dedicated debate prep this weekend under the tutelage of several high-profile Republicans, including former New York Mayor Rudy Giuliani. Under their guidance, Trump has continued to refine his “America first” stances and promote proposals such as deporting undocumented immigrants, racially profiling Muslims to prevent terrorist attacks, the use of “stop-and-frisk” policing to reduce crime in the inner cities.

6. WHO ARE THE THIRD-PARTY CANDIDATES?

Gary Johnson and Jill Stein were not invited to the debate. PHOTOS: AFP, REUTERSGary Johnson, the Libertarian candidate, and Jill Stein, the Greens Party nominee, were not invited to the debate. They did not reach the threshold, set by the Commission on Presidential Debates (CPD), of 15 per cent support in five national opinion polls.

Johnson's support is currently at 8.9 per cent and Stein's at 2.9 per cent, according to an average of recent surveys.

7. WHERE CAN YOU WATCH IT ONLINE?

The university will provide a live-stream of the debate here. Other sites where you can catch the debate include Live News Chat and C-SPAN.

8. HOW MANY PEOPLE WILL BE WATCHING?

The current record for a televised presidential debate is 80.6 million viewers, set by the 1980 encounter between the Democratic incumbent, Jimmy Carter, and his Republican challenger, former California governor Ronald Reagan. Many analysts expect that to be broken.

According to a Reuters/Ipsos poll released on Monday, half of America’s likely voters would rely on the debates to help them make their choice. More than half, 61 per cent, were hoping for a civil debate and were not interested in the bitterness shown on the campaign trail.

9. WHEN WILL THE OTHER DEBATES BE HELD?

The second debate is scheduled for Oct 9 at Washington University in St. Louis, and will take the form of a town-hall meeting. The third and final debate will be held on Oct 19 at the University of Nevada, Las Vegas, and will have the same format as the first debate.